Archive for the ‘polarization’ Category

Planck sees the Crab

September 29, 2009

[cross-posted from Planck UK site]

Last week, the Planck satellite observed the Crab Nebula, also known as M1 from Messier’s catalogue, and Tau A to radio astronomers. The Crab Nebula is one of the brightest objects in the sky at radio and sub-mm wavelengths, and is used by many astronomers to calibrate their instruments. It is the remnants of a star which underwent a violent and catastrophic explosion at the end of its life, an event known as a supernova. The Crab Nebula is associated with one of the few supernovae which was observed, as it was seen by Chinese and Arab astronomers in 1054.

Hubble image of the Crab Nabula
Hubble Space Telescope image of the Crab Nebula, showing the colourful nebula and filaments. Image credit: NASA/ESA

After the supernova, the outer layers of the star were scattered widely into a huge cloud of gas, which we now see as a nebula, and is frequently observed by amateur and professional astronomers. The core of the star ended up as a pulsar, a rapidly rotating, magnetised neutron star. However, it is the surrounding nebula that Planck is interested in. The nebula is around 13 lightyears across, which is around 10,000 times larger than our solar system (depending on where you define the edge). It is also relatively close in astonomical terms, at about 6 lightyears distance, and so is also relatively large in terms of astronomical objects see from Earth. It is around 7 arcminutes across, which is 7/60th of a degree, or about 1/4 the diameter of the full moon as seen from Earth. The smallest of Planck’s beams on the sky is 5 arcminutes and the largest 30 arcminutes, so to the majority of Planck’s detectors the Crab Nebula will have appeared as a “point source” – a single blip in the data. However, it is so bright that it would still be very obvious.

SCUBA measurements of the Crab nebula
SCUBA image of the Crab Nebula at 350GHz. The colour shows the brightness (blue/black=dark, red/white=bright), and the length og the black lines shows the relative amount of polarisation. Image credit: J.S. Greaves, W.S. Holland & T. Jenness (Joint Astronomy Centre)

Because the central pulsar is magnetised, the Crab Nebula is permeated by a magnetic field. This causes any electrons in the nebula, of which there are many, to spiral around the magnetic field lines and emit radiation known as synchrotron radiation. This radiation, which is brightest at radio wavelengths, but still very strong at Planck frequencies, is polarised, which makes the Crab an ideal object for calibrating the polarisation-sensitive detectors on Planck. Many experiments have observed its polarised radiation, including the SCUBA instrument on the JCMT on Hawaii, which observed it at 350GHz (a wavelength of 850 microns). One of the properties of synchrotron radiation is that it varies in a fairly predictable way with frequency, so it is possible to tell from an observation at one frequency what it will look like at another.

The data will take time to analyse, but will be ingested into the Planck database and used to help calibrate its instruments. The Crab Nebula is not the only calibration source, but is the best single source in the sky for calibrating the polarisation angles.  Other non-polarised sources include the planets in our own solar system, particularly Jupiter, which are very bright at Planck’s frequencies as well as to the naked eye.

Bad news in CMB science: Clover Cancelled

April 5, 2009

I was shocked to read on Nature’s news pages that the Clover project has been cancelled. This is a UK project aimed at probing the polarization of the microwave background and was capable of unveiling the physics of the inflationary epoch of the universe. A lot of money had already been spent on the project, including the design and construction of most of the instrument. All that remained was shipping and installation in Chile, and running costs. But this has proved too much for STFC, the relevant funding council.

This is a real pity The project was well advanced, well justified and had the promise of producing some spectacular science. For the paltry sum, these days, of 2.78 million it’s been thrown away, along with the years of effort to get it close to deployment.

At a time when STFC is planning on spending tens of millions of a probe that will launch poles into the moon, a branch of science where the UK has no track record, this is appalling news for physics and shameful action by the funding council.

It also bodes ill for proper funding for Planck exploitation in the UK.

We were beginning to think that we’d seen the worst of the physics funding crisis that blew up 18 months ago. It looks like the effects are far from over.

Always looking forward

July 18, 2008

Planck hasn’t launched yet, and it’s a long long way before its results will come out, but it’s the way of space missions to already be thinking of the next step.

BPol was recently proposed to ESA, unsuccessfully, and I’m now at a meeting in the US where a number of possible future CMB satellites are being discussed. A launch in the 2020s is expected. There are also a host of ground-based and balloon borne projects planned between now and then.

Planck will draw the line under temperature fluctuations. The current target is polarization which, it is hoped, can say something about the the inflationary epoch in the first 10-30 seconds after the Big Bang. Exciting stuff, even if it is going to take us a long time to see what the results are!